When it comes to thermal storage tanks there are many shapes, sizes and pressure ratings available. These range from tanks that resemble (or are) domestic water heaters to larger ASME-certified pressure vessels.
In most hydronic thermal storage applications it’s desirable to maintain temperature stratification within the tank (e.g., hottest water at the top, coolest water at the bottom). Good stratification improves the “quality” of thermal energy available from the tank, relative to that available from a full mixed tank.